The present invention relates to a steering system for an all-terrain crane truck or like automotive vehicle and, more particularly, to an improvement in such a steering system which includes one steering wheel accessible from a seat adapted to drive the vehicle and another steering wheel accessible from a seat located behind the first-mentioned seat and adapted to manipulate a crane or the like, so that one may steer the vehicle at a desired one of the two seats.
Automotive vehicles designed for special industrial applications such as construction and engineering works include an all-terrain crane truck which is provided with a crane on top of its chassis. One may manipulate the crane while moving the truck from one place to another. An all-terrain crane truck is usually equipped with two spaced seats on its chassis, i.e., a seat positioned in a front portion of the chassis for driving the truck on throughfares and other ordinary roads (hereinafter referred to as a driver's seat for convenience), and a seat positioned in a rear portion of the chassis for manipulating the crane (hereinafter referred to as a craneman's seat for convenience). The crane is free to swivel in a horizontal plane while lifting a desired object and, for the convenience of a person who manipulates the crane, the craneman's seat is constructed to rotate together with the crane.
At a construction cite, for example, while one is often required to relocate the crane truck during the course of operation of the crane, it would be awkward for him or her to move from the craneman's seat at the rear to the driver's seat at the front every time the need for relocation of the truck arises. In light of this, it has been proposed to furnish the craneman's seat with the same driving instrumentation as a one associated with the driver's seat, such as a steering wheel, gear shift lever, accelerator pedal and brake pedal, so that one may steer the truck at the rear craneman's seat as well as from the front driver's seat. Another capability of such a special truck is a so-called four-wheel steering mode operation in which not only front wheels but also rear wheels are steered in order to facilitate sharp turns in narrow places at construction cites.
For the structures described above, a reference may be made to Japanese Utility Model Application No. 196480/1982, for example.
Where an automotive vehicle has two steering wheels such as those assigned to the front driver's seat and rear craneman's seat as described to allow the vehicle to be steered at any of the seats, it is desirable that while the vehicle is steered at one of the seats, steering at the other seat is inhibited. The reason is that should the vehicle be steerable at the rear craneman's seat while it is steered at the front driver's seat, its running direction might possibly be changed against the driver's intention to bring about a dangerous situation.
The steering system installed in a vehicle of the kind described comprises a hydraulically operated power steering system. In such a steering system, disabling steering operations at the craneman's seat while the vehicle is steered at the driver's seat may be accomplished by switching hydraulic circuits associated with power cylinders such that the hydraulic pressure communicated to the power cylinders cannot be controlled by manipulation of the steering wheel at the craneman's seat so long as steering at the driver's seat is under way. This hydraulic circuit switching scheme is not fully acceptable, however, because the hydraulic circuits still have a chance of being switched at the craneman's seat while the vehicle is steered at the driver's seat.
As previously stated, the vehicle of the kind concerned may be steered at both the front and rear wheels to reduce the turning radius and, thereby, enhance operationability of the vehicle. The four-wheel steering mode, however, has the drawback that when effected during high-speed running on ordinary roads it might accidentally cause the vehicle to make a sharp turn resulting in an unstable position of the vehicle body and, therefore, unstable steering. It is desirable, therefore, that a four-wheel steering mode be enabled only at low vehicle speeds with a two-wheel steering mode as distinguished from the four-wheel mode set up at high vehicle speeds.